1. Field of the Invention
The present invention relates to a horizontal multistage press in which a plurality of hot plates are disposed between frames provided on opposite sides in face-to-face relation so that they can freely be opened and closed and sheets to be processed, such as pieces of plywood, decorative sheets, and single-sheet veneers are heat-pressed.
2. Description Of the Related Art
In general, a multistage press of the type which is mainly employed during plywood manufacturing processes includes a movable platen capable of freely ascending and descending by means of ram cylinders disposed in a lower frame and a plurality of stages of hot plates provided between an upper frame and the lower frame. The apparatus of this type is arranged in such a manner that, after sheets to be processed have been inserted into the gaps between the respective hot plates, the press is closed by moving the movable platen upwardly to lift up the hot plates step by step from the lowermost hot plate to the uppermost hot plate, thereby obtaining processed sheets by the application of pressure and heat for a predetermined period of time.
This kind of multistage press having a plurality of hot plates disposed one above another in the vertical direction is constructed in such a manner that support lug attached to the four corners of the hot plates are respectively mounted on corresponding ladder-like support members attached to associated columns such as to support the aforesaid hot plates. Therefore, when the thicknesses of the respective hot plates are to be determined, it is necessary to take into consideration the level of strength which the hot plates are required to have in heating and pressing unprocessed sheets and which will be sufficiently resistant to the levels of deflection and curvature formed by the distance between the supporting points of the hot plates. Consequently, the hot plates must have a thickness which is more than would be necessary simply from the viewpoint of rigidity.
In addition, since such hot plates which have a thickness that is more than is necessary and, hence, an increased weight are stacked in a multistage manner, the levels of pressure applied to the sheets processed by means of the hot plates greatly vary as between the uppermost and lowermost plates. This causes imperfect bonding between the elements of a processed sheet and results in its thickness being insufficient. In addition, it is necessary to increase the diameters of the ram cylinders which are the means for moving up and down the respective hot plates, as well as the capacity of the pump employed to supply hydraulic fluid to the cylinders, and this requires a hydraulic-pressure unit of enormous size. As a consequence, the size of the multistage press per se is increased.
For this reason, attempts have been made to correct variations in pressure levels, to eliminate imperfect bonding in sheets pressed and in combination with heat, and to reduce the weight of sheets or the size of a hydraulic pressure unit by employing a horizontal multistage press having upright hot plates arranged side by side, instead of the previously-described vertical multistage press in which a plurality of hot plates are arranged one above another. However, the above-mentioned horizontal multistage press typically requires a device for loading upright sheets to be processed in the gaps between the hot plates and unloading upright processed sheets from therebetween, such hot plates being equally spaced apart in side-by-side relation during the period in which the press is at a standby.
In the light of the aforesaid circumstances, the present applicant achieved a certain degree of improvement in the invention described in Japanese Patent Application No. 109118/1984 and Japanese Patent Laid-open No. 250901/1985, entitled "APPARATUS FOR LOADING AND UNLOADING SHEETS FOR USE WITH HORIZONTAL MULTISTAGE PRESS" and filed by the applicant of the present invention. Referring to the construction of the prior-art apparatus, chain conveyors are disposed at the side of the horizontal multistage press into which unprocessed sheets are loaded and on the other side of the same from which processed sheets are unloaded, the axes of such chain conveyors crossing at right angles the directions in which the sheets are loaded and unloaded. Belt conveyors are disposed parallel to said conveyors with the levels of their upper conveying surfaces higher than those of the chain conveyors. These conveyors are controlled to run at substantially equal speeds, and loader or unloader rack assemblies are secured to the aforesaid chain conveyors, such rack assemblies each having racks at least greater in number than the total numbers of hot plates and being in phase with the associated gaps between the hot plates when they are placed upright in side-by-side relation during the period in which the press is at a standby. In addition, sheet loading and unloading conveyors are disposed and adapted to move up and down relative to the level of the upper conveying surfaces of the aforesaid belt conveyors, thereby enabling the loading and unloading of sheets. This arrangement enables the loading of sheets to be processed from the loader racks into the gaps between the hot plates and the unloading of processed sheets from the hot plates into the gaps between the unloader racks during the period in which the press is at a standby, thereby attaining a reasonably successful result.
However, each of the individual single-sheet veneers which are combined to form an unprocessed sheet includes to some extent defects such as unevenness, curvature and twisting. Furthermore, certain kind of raw lumber manifests these phenomona conspicuously during the cutting of the raw lumber into single-sheet veneers or shortly after the cutting. These single-sheet veneers are superposed upon each other and temporarily bonded to form a sheet to be processed. When the thus-obtained unprocessed sheets are to be loaded from the loader racks into the gaps between the hot plates by means of driving the sheet loading and unloading conveyors with their sheet carrying surfaces flush with each other, the leading ends of the unprocessed sheets may collide with the insertion guides that are formed on the associated ends of the hot plates. Even if the leading ends are successfully inserted into the gaps between the hot plates, the unprocessed sheets may get blocked at intermediate portions of the passages before they are completely loaded in place between the hot plates, so that it is impossible to completely heat them.
There is also raw lumber of the kind in which, as described above, a camber is formed on processed sheets which have been heat-pressed by the stress produced by heating. Even if the processed sheets clamped between the hot plates are temporarily lifted above the sheet carrying conveyor by the lifters and are to be loaded in the unloader racks in cooperation with the sheet loading and unloading conveyors, the sheets may get blocked in the gaps between the hot plates. Or may collide with the unloader racks, so that it is impossible to completely unload the processed sheets.
The loader and the unloader for unloading and loading the sheets are generally disposed close to each other. However, since they need to be separately driven, the loader and the unloader are fixedly disposed at a predetermined interval. Therefore, in order to transfer the unprocessed sheets from the loader racks to the hot-plate side and the processed sheets from the hot-plate side to the unloader racks, the gaps between the loader racks, the hot plates, and the unloader racks must be arranged in phase.
However, the loader or unloader provided in the related arts has a tendency to involve discrepancies between the respective racks and the hot plates by reason of mechanical errors caused by aging or thermal expansion of members that support the respective racks. Thus, the unprocessed and processed sheets may collide with the ends of the hot plates and the unloader racks, respectively.
In addition, if the sheets to be processed display some degree of camber, there is some risk of them colliding with the ends of the unloader racks. In particular, in cases where the unprocessed plywood sheets are temporarily bonded and each single-sheet veneer which constitutes the plywood is of an irregular shape at the four edges and hence easily exfoliated, when the plywood sheet is loaded, a portion thereof may be bent by contact with one end of the hot plate. If the sheet is heat pressed in the bent state, the result is to produce defective products.
Moreover, during the maintenance of the loader and unloader or when small pieces of lumber and dust accumulated during the operation are to be removed, the close disposition of the loader and unloader results in hindrance to such tasks as cleaning.
Normally, when a plurality of cylinders are to be disposed in the lower frame of a vertical multistage press, the movable platen is held to allow the rams to be placed at substantially the center of the hot plates, whereby well-balanced pressure is applied to the substantial center thereof. The sheets to be processed are pushed at their trailing ends by pushers, inserted into the gaps between the hot plates, and respectively centered on the hot plates. Thus, the pressure point of heat pressing is located substantially at the center of the respective sheets to be processed, so that well-balanced pressure can be applied over the whole of the sheet from the center to the four edges. Also, according to this mechanism, even if there is a difference between the sizes of unprocessed sheets, the sheets can be centered on the hot plates by adjusting the working range of the pushers as the occasion demands, thereby providing the aforesaid well-balanced pressure.
However, in general, a horizontal multistage press is arranged in such a manner that sheets to be processed are not overlaid on the hot plates which are placed upright in side-by-side relation, but the unprocessed sheets are inserted into the gaps between the hot plates in a substantially upright manner with the lower ends of the sheets being held. It is therefore impossible to lift the lower ends of the unprocessed sheets up to a height more than the level of the lower ends of the hot plates. As a result, when the length and width of the unprocessed sheets are relatively enlarged or reduced, the respective pressure positions in the vertical direction and in the direction of travel of the unprocessed sheets are shifted and the pressure balance between the center and the four edges are lost. In addition, the processed sheets involve the drawbacks such as reduced thicknesses thereof and exfoliation attributed to unproper application of pressure.
In general, a typical press is arranged in such a manner that water vapor, hot oil or other heating media is supplied to the interiors of the respective hot plates, and is maintained at temperatures between 110.degree. and 180.degree. C., depending on the kind of each sheet to be processed.
Right and left frames and movable platen as essential constituent members for a multistage hot press are reinforced by beams disposed in their hollow internal spaces in a welded manner, and they are respectively shaped in the form of a sealed box. Since the members are always heated by temperatures provided by the hot plates, a thermal strain occurs therein. This forms a cause of producing hindrance to the opening and closing of the hot plates and reduction in the thicknesses of the sheets which have been processed.
For this reason, the frames and the movable platens are constructed as shown in FIGS. 41 and 42 in such a manner that rib plates 130 are combined longitudinally and laterally at predetermined intervals so as to constitute a lattice, and in addition side plates 131 are disposed on the edges of the thus-obtained structure, the respective contacts in the structure being united with each other by welds 132. The structure is formed into a lattice-like structure 133 having opposite open ends thereof in the direction of application of pressure, that is, on the right and left sides, so as to prevent overheating by air insulation effects.
It is true that this lattice-like structure 133 provides a certain degree of cooling effects as compared with complete box type structures; however, it is impossible to suppress strain stress which might be caused by the thermal expansion of the aforesaid movable platens during a heating step.